The present invention relates to a system for permanently mooring a tanker ship on high seas by fixing it to a structure which emerges from the sea surface and has the following characteristics:
the fixed structure is relieved from all of the motions of the ship and must thus bear only low frequency forces in addition to the shifting of the charging lines from the bottom to the surface so as to permit the use of rigid lines PA1 the recoil force is represented by ballast chambers or by a solid ballast PA1 nonfixed ("compliant", thus adaptable) structures are known which are capable of permanently mooring tanker ships. PA1 (A) Towers with one or more articulated members to which the tanker ship is moored by a rigid arm; PA1 (B) Buoys anchored to the sea bottom by one or more catenary lines and connected to the ship by a rigid arm; PA1 (C) Catenary line systems directly connected to the ship's bow and supported by the ship herself.
These structures can be divided into:
A shortcoming of all these approaches is the necessity of employing flexible hoses for the flow of the oil. In the approaches (A) in order to overtake the articulated sections, in the (B) and (C) cases due to the lack of a fixed structural supporting member and also to allow the other flow lines to follow the ship's motion. Such flexible connections, especially when deep waters are encountered, are difficult to check and to maintain and require a periodical replacement.
The system according to the present invention does away with such drawbacks since it becomes possible to use rigid lines supported by a fixed structure and capable of transferring oil from the bottom to the surface.
The (A) and (B) systems, in addition, require high buoyancy volumes in the vicinity of the surface, the former system imparting stiffness to the assembly, to develop the recoil force and the latter system supporting the weight of the chains. On these volumes, both due to their superficial position or their position close to the sea surface and their dimensions, considerable wave forces and current are exerted thereto which load the entire structure and influence their dimensions.
The system described herein develops, on the contrary, its recoil force by exploiting ballast chambers which, due to their reduced size and their possibility of being positioned also in deep water are cheaper and are exposed to negligible environmental forces.
Also the dimensions of the foundations will become cheaper due to the fact that ballast chambers have been selected instead of buoyancy chambers.
The (C) systems, having no buoyancy chambers or buoyant component parts, because the weight of the anchoring cables is supported by the ship itself, do not present, obviously the latter mentioned drawback but it should nevertheless be noted that by such systems all the high-frequency motions of the ship are transferred to the anchoring cables and to the flow lines so that all these lines are subjected to considerable dynamic overloads. A description is now given of an embodiment of the present invention by way of example only and without limiting the possibility of introducing modifications which do not change the substance of the invention, reference being had to FIGS. 1, 2 and 3, wherein: